1. Field of the Invention
The present invention generally relates to projection exposure apparatuses used in the fabrication of microstructured devices. More particularly, the invention relates to a method of determining which lenses should be made of fluoride crystals such as calcium fluoride (CaF2), and which measures can be taken against adverse effects caused by intrinsic birefringence.
2. Description of Related Art
Projection exposure apparatuses are commonly used in the fabrication of electrical and optical integrated circuits for forming images of device patterns on semiconductor substrates. More particularly, the process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, a pattern is formed on the photoresist using a projection exposure apparatus, such as a step-and-scan tool.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. A mask (also referred to as a reticle) containing a pattern to be formed on the photoresist is illuminated by the illumination system. During exposure, the projection objective forms an image of the mask onto the photoresist. After developing the photoresist, an etch process transfers the pattern into a patterned thin film stack on the wafer. Finally, the photoresist is removed.
Since the resolving power of the projection objective is inversely proportional to the exposure wavelength, new generations of such tools generally use exposure light with a shorter wavelength than used by tools of the previous generation. At present, deep ultraviolet light having a wavelength of 248 nm or 193 nm is used for submicron lithography. The next generation of photolithography tools will use exposure light having a wavelength of 157 nm.
One of the major problems encountered when using exposure light having such short wavelengths is the fact that conventional lens materials such as quartz glasses are not sufficiently transparent in the deep ultraviolet wavelength domain. A low transparency reduces the brightness of the image and results in increased heating of the lenses. Lens heating, in turn, often causes undesired variations of the shape of the lenses and also of their index of refraction. Apart from that, DUV projection light frequently interacts with quartz glasses such that their density and thus their index of refraction are altered irreversibly.
For that reason, other materials have been investigated that do not suffer from the deficiencies described above.
Among the most promising materials that can replace conventional lens materials is a class of single crystal fluoride materials that have, for the wavelengths of interest, much higher transmittances than conventional lens materials. Thus far, calcium fluoride (CaF2) seems to be the most promising candidate within this material class; other cubic crystals belonging to that class include barium fluoride (BaF2), lithium fluoride (LiF2), strontium fluoride (SrF2) and isomorphous mixtures such as Ca1−xBaxF2.
Of prime concern for the use of these cubic crystalline materials for optical elements in deep ultraviolet lithography tools is their inherent anisotropy of the refractive index. This inherent anisotropy is commonly referred to as “intrinsic birefringence”. Since the intrinsic birefringence scales approximately as the inverse of the wavelength of light, the issue of birefringence becomes particularly significant if the exposure wavelength is below 200 nm.
In birefringent materials, the refractive index varies as a function of the orientation of the material with respect to the direction of incident light and also of its polarization. As a result, unpolarized light propagating through a birefringent material will generally separate into two beams having orthogonal polarization states. When light passes through a unit length of a birefringent material, the difference in refractive index for the two ray paths will result in an optical path difference or retardance. The retardance causes wavefront aberrations that are usually referred to as “retardance aberrations”. These aberrations are capable of significantly degrading image resolution and introducing distortion of the image.
One of the most interesting approaches for addressing the problem of retardance aberrations is to combine several cubic crystals whose crystal lattices are oriented with respect to each other in such a way that the overall retardance is reduced by mutual compensation. The underlying idea is to exploit the fact that, if a first polarization state is retarded in one crystal, a second polarization state being orthogonal to the first one may be retarded in another crystal of the optical system. As a result, the retarded wavefront of the first polarization state may “catch up” with the wavefront of the second polarization state while the latter is retarded in the other crystal. The overall net retardance of both crystals, i.e. the difference between both retardances imposed on the different polarization states, may then be considerably reduced or even made to vanish.
In US 2004/0105170 A1 an optical system is described comprising two groups each including two lenses that are made of cubic crystals. In one group, two crystals are oriented such that each [111] crystal axis (or an equivalent crystal axis such as the [11-1] axis, for example) coincides with the optical axis that is defined as the symmetry axis of the optical system. The orientations of the crystal lattices of both crystals differ in that the crystal lattice of one of the crystals results from rotating the crystal lattice of the other crystal around the optical axis by 60°. As a result of this rotation that is sometimes referred to as “clocking”, the rotational asymmetry of birefringence inherent to each single crystal is substantially reduced if taking the group as a whole.
Within the other group, the two lenses are made of crystals whose crystal lattices are oriented such that each [100] crystal axis coincides with the optical axis of the optical system. Again, the crystal lattices are rotated around the optical axis, but in this case by only 45°. Also in this group the birefringences of both crystals combine such that the overall birefringence of the group is almost rotational symmetrical.
However, since the birefringences induced in both lens groups have different signs, different polarization states are retarded in each group. This opens the way for mutually compensating the effects of birefringence induced in both lens groups. Since the birefringence in both lens groups not only differs in sign, but approximately equals in magnitude, the overall retardance can be significantly reduced if both polarization states travel in the same direction and with the same path lengths within each crystal.
Often the design objective is not (or not exclusively) the reduction of overall retardance, but to positively affect the retardance or its angular pupil distribution for achieving other advantageous effects. For example, in many cases it is more desirable to have a particular symmetric angular retardance distribution than to achieve a minimum mean retardance. Therefore the optimum crystal lattice orientations depend on the specific design objective.
Unfortunately, the path lengths and directions of the rays through the lenses cannot be varied just at it would be required for achieving the desired retardance property. This is because the shape of the lenses, their arrangement within the optical system and thus also the optical paths taken by light rays when propagating through the lenses are almost completely determined by the design of the optical system as a whole in view of the imaging properties that are to be achieved.
Until now, more or less heuristic approaches have been followed when it was to determine which lenses should be made of a fluoride crystal and, if there are any such lenses, which measures can be taken in order to achieve tolerable aberrations caused by intrinsic birefringence.